The present invention concerns a self-fed commutation device sensitive to a temperature gradient.
This device is particularly suitable to the commutation of measuring transducers to thermocouples utilised for measuring the temperatures of combustion of aircraft turbojet engines.
In these engines, it is advisable to measure the temperature of combustion of gas with the greatest precision, to keep the engine at its optimum output.
Because the exhaust of burnt gases occurs in a section formed as a ring, a series or "harness" of temperature transducers, usually thermocouples, is generally used, with two to twelve in number distributed circularly in the ring and connected in parallel in such manner that the common voltage at the point of connection of all the thermocouples represents substantially a measurement of the mean of the temperatures in the gas exhaust ring. French Pat. No. 2,382,000 describes such a harness, for which it proposes a method of compensation of different thermocouples in a manner to give to each thermocouple branch the same impedance. The compensation thus always remains balanced whatever the variations of the temperatures.
It should be noted from the above that the exhaust temperature of the gases is measured, and not the temperature of combustion, which is the parameter to be monitored. This results because it is not possible to place measuring probes in the very middle of the combustion chamber of the engine (referenced 1 on FIG. 1 which represents a schematic sectional view of an engine). They can be placed only in the proximity of this chamber, for example, in a housing made in a fixed blade downstream of the combustion chamber. Reference 10 designates one of the probes of the harness installed in this way.
Generally, measurement of the exhaust temperature of the burnt gases is a good representation of the temperature of combustion.
This is not however true in certain particular configurations of flight, particularly during reverse flows: in this case, the bored fixed blades duct, at several levels, fresh air into the passage 2 situated between the inner and outer casings 3, 4 of the engine for re-injecting it into the path of the burnt gases. The reversal of direction of circulation of the ducted gases involves a sharp fall at the position of the harness of thermocouples 10 (a fall of several tens of degrees for an exhaust gas temperature of the order of 950.degree. C.), whilst the temperature of combustion remains substantially the same.
Pilots are warned of this phenomenon and know that, during reverse flows, it is not advisable to take account of sharp falls which appear on the temperature indicators of the instrument panel.
In addition, it is known that there exists--for this configuration of flight but for it only--another position for the thermocouples at which the temperature of the exhaust gases is a good representation of the temperature of combustion.
However, the installation of temperature measuring circuits in existing engines does not permit leading out of more than two conductors. It is just not possible, if one wishes to adapt existing engines to double measurement, to arrange two thermocouple harnesses (which will be placed for example at 10 and 20 in FIG. 1) with two outputs connected to two distinct indicators giving the temperature at one or the other position in the engine, or through a commutation device functioning from a signal of the configuration of flight given as a function of the controls set by the pilot.